The present invention relates in the first place to a device for the unidirectional locking of a cylindrical rod of a linear actuator, according to the preamble of claim 1.
A device according to the preamble of claim 1 is known from document WO 97/30291, in which the concave intermediate portions of the diabolo-shaped rollers are in rolling engagement with a cylindrical rod of the actuator.
In this known device, the wedging surfaces defined within the outer body of the actuator are formed by flat converging tracks on which the cylindrical end portions of the rollers roll.
In this known arrangement the rollers are subjected to bending stress by the wedging forces as in the case of a beam supported at both ends (the cylindrical portions of the rollers) and loaded in the middle (on the concave intermediate portion of the rollers).
Such bending stress has sometimes given rise, in experimental use, to crackings and breaks of the rollers in the most restricted section of the diabolo shape.
Such crackings and breaks can be avoided by making the rollers of steel materials, with quenching treatments having a high cost which it would be desirable to reduce.
Being aware of the danger of cracking and breakage of the rollers, the latter require expensive quality controls before assembling, in order to allow to discard as from the beginning the rollers which show, in their intermediate section, cracks or other defects which could constitute stress raisers.
The main object of the invention is precisely to provide a locking device of the type considered, which allows to use diabolo-shape rollers made of little expensive materials, even different from steel materials, for example of synthetic material, thanks to the absence of bending stresses in such rollers.
According to the invention, this object is achieved by means of a locking device as claimed.
In a locking device according to the invention the rollers are subjected only to diametral compression forces between two convex wedging surfaces, on the one hand the surface of the rod and, on the other hand, the corresponding wedging surface within the fixed outer body.
As one understands, a body of revolution subjected to compression stress by wedging forces acting thereon only diametrically is able to withstand such forces much better than the same body would do when subjected to bending stress, under equal force conditions.
This allows to choose for the rollers, under equal force condition, a steel of lower quality or even, in certain cases, a synthetic material having a good resistance to both compression and wear, such as for example a polytetrafluoroethylene.
When rollers of the same steel material as those of the known prior art are used, the claimed arrangement allows to obtain a locking device having a higher factor of safety.
Further, given that possible surface defects of the rollers have no influence on their ability to resist to compression, also the quality controls which in any case have to be carried out on the rollers before assembling may represent a lower cost.
The invention also relates to a unit for the bidirectional locking of the sliding of the rod of a linear actuator, which includes a pair of locking devices as claimed, as well as to a linear actuator which includes one of said devices or said unit.
Throughout the present description and in the claims, the terms xe2x80x9caxialxe2x80x9d, xe2x80x9cradialxe2x80x9d, xe2x80x9cchordalxe2x80x9d, their derivatives, as well as similar terms, indicate directions with reference to the axis of the actuator rod.